1. Field of the Invention
The present invention relates to a system for providing data protection in ring type networks and other networks. The present invention also relates to a redundant receiver architecture for use in a fiber optic transmission system.
2. Description of the Related Art
A basic point-to-point fiber optic transmission system consists of three basic elements: the optical transmitter, the fiber optic cable and the optical receiver. The optical transmitter converts an electrical analog or digital signal into a corresponding optical signal. The optical receiver converts the optical signal back into a replica of the original electrical signal.
Optical signals generally undergo a 0.2 dB loss per kilometer when transmitted over a fiber optic channel. This translates to a 4% loss of the light regardless of modulation. Since the receiver signal can be quite small, receivers often employ high gain internal amplifiers. FIG. 1 is a functional diagram of a known simple analog optical receiver. The first stage 1 is an operational amplifier connected as a current-to-voltage converter. This stage takes the tiny current from the photodiode 2 and converts it into a voltage, usually in the millivolt range. The next stage 3 is a simple operational voltage amplifier in which the signal is raised to the desired output level 4.
FIG. 2 is a functional diagram of a simple digital optical receiver. As in the case of the analog receiver, the first stage 1 is a current-to-voltage converter. The output of this stage, however, is fed to a voltage comparator 10, which produces a clean, fast rise-time digital output signal. The trigger level adjustment 11, when it is present, is used to touch up the point on the received signal where the comparator 10 switches. This allows the symmetry of the recovered digital signal to be trimmed as accurately as desired. Additional stages may be added to receivers to provide drivers for coaxial cables, protocol converters or a host of other functions in efforts to reproduce the original signal as accurately as possible.
When a signal is compared to a reference voltage, however, the system does not guarantee that information is in fact received. That is, the receiving device must ultimately detect the presence of missing information and determine how to obtain it. Accordingly, to handle missing information, redundant transmission of data over the same fiber optic channel is typically employed. This causes delays in data transmission and adds to the complexity of the transmission and reception equipment.